The present invention relates to multicomponent fibers comprising starch and polymers, in particular, where a starch component has been at least partially insolubilized by exposure to an insolubilizing agent initially present in a polymer component of the fiber. The fibers can be used to make nonwoven webs and disposable articles.
There has not been much success at making starch containing fibers on a high speed, industrial level due to many factors. Because of the costs, the difficulty in processing, and end-use properties, there has been little or no commercial success. Starch fibers are much more difficult to produce than films, blow-molded articles, and injection-molded articles containing starch. This is because of the short processing time required for starch processing due to rapid crystallization or other structure formation characteristics of starch. The local strain rates and shear rates are much greater in fiber production than in other processes. Additionally, a homogeneous composition is required for fiber spinning. For spinning fine fibers, small defects, slight inconsistencies, or non-homogeneity in the melt are not acceptable for a commercially viable process. Therefore, the selection of materials, configuration of the fibers, and processing conditions are critical. In addition to the difficulty during processing, the end-use properties are not suitable for many commercial applications. This is because the starch fibers typically have low tensile strength and are sticky.
To produce fibers that have more acceptable processability and end-use properties, it is desirable to use non-starch thermoplastic polymers in combination with starch. The melting temperature of the thermoplastic polymer should be high enough for end-use stability, to prevent melting or undue structural deformation during use, but low enough so that the composite fibers are processable with starch.
There exists today an unmet need for cost-effective, easily processable, and functional starch-containing fibers that also have acceptable water resistance. Although methods exist for rendering thermoplastic compositions containing starch more insoluble by, for example, cross-linking such as in U.S. Pat. No. 6,218,532, Apr. 17, 2001 to Mark et al., such crosslinking adversely affects the processibility of starch bicomponent fibers. The fibers produced by Mark et al. are crosslinked before processing, thereby limiting their processability and their overall ability to be produced in small diameters. U.S. Pat. No. 5,874,486 to Bastioli et al., Feb. 23, 1999, relates to polymeric compositions comprising a matrix including a starch component and a thermoplastic polymer in which a high level of filler is dispersed in starch. U.S. Pat. No. 5,844,023 to Tomka, Dec. 1, 1998, relates to a polymer dispersion consisting essentially of starch dispered as a discontinuous component and at least one specific polymer.
The present invention addresses the problem of mass loss of starch from the starch component of a multicomponent fiber in the presence of water.
The present invention is directed to melt spinnable multicomponent fibers comprising a first component and a second component. The first component comprises a starch insolubilizing agent and a thermoplastic polymer and the second component comprises destructurized, typically, thermoplastic starch. The insolubilizing agent acts on the starch of the second component to render the starch less soluble when the fiber is exposed to water. Such interaction may include diffusion of the insolubilization agent from the first component across the interface to render neighboring starch regions insoluble, may include diffusion of the insolubilization agent throughout the second starch component to reach an equilibrium of agent throughout the fiber, a diffusion gradient thereformed, or may include chemical reactions with the starch, for example. The resultant fiber loses less starch when in contact with water than a similar fiber without the insolubilization agent. A difficulty with adding the insolubilization agent to the second component during processing is that such a composition has very poor spinnability. An embodiment of the invention is the resultant fiber after action of the insolubilizing agent on the starch of the second component. Such a fiber comprises a second component which comprises destructured insolubilized starch or, typically, thermoplastic insolubilized starch.
The configuration of the multicomponent fibers can be sheath-core, islands-in-the-sea, side-by-side, segmented pie, for example, or various combination thereof. In the embodiments where starch is present in the component potentially having contact with water, i.e., the sheath of a sheath-core configuration, for example, soluble starch can be removed upon contact with water. However, in such a configuration, insolubilized starch can remain in the sheath component to form a coating around the core component.
Such compositions are cost-effective, suitable for use in commercially available equipment, while possessing a significant amount of the total composition that is biodegradable. Fibers of the present invention have a higher wet strength and lower water solubility than existing fibers. The resultant at least partially insolubilized starch of the multicomponent fibers of the invention has less starch loss when placed in contact with water as compared to existing fibers. The present invention is also directed to nonwoven webs and disposable articles comprising said multicomponent fibers. The nonwoven webs may also contain other synthetic or natural fibers blended with the fibers of the present invention.